1) Field
The disclosure relates to methods in the design and manufacture of composite structures and parts, and in particular, relates to a composite stack analysis method for use in the design and manufacture of composite structures and parts that evaluates the interactions of plies at the design and manufacture levels.
2) Description of Related Art
Composite structures and parts are used extensively in aircraft, aerospace, and other applications in which materials having a high strength-to-weight ratio are necessary. Composite structures may be manufactured, either manually or using an automated apparatus, by progressively building up the structure with a plurality of layers of thin composite tape or tow (i.e., untwisted bundles of continuous filaments, such as carbon or graphite fibers, preimpregnated with a thermoset resin material such as epoxy) laid one layer upon another and laminated together. These layers are often referred to as partial or full plies. For large structures exceeding the available material widths, or designs that can benefit greatly from unidirectional stress paths, each ply layer is typically made up of multiple tape strips or tow courses of material placed near or abutting edge-to-edge next to each other or overlapped to some extent. In order to analyze the ply as if it were one continuous piece, it is generally advantageous to reduce the gap and overlap distances or tolerances between the tape strips or tow courses. For example, in certain relatively high technology industries such as the aerospace industry, the gap distance may be held to 0.10 inch or less.
Prior to the actual manual or automated manufacturing of the composite structure, the composite structure comprised of partial or full plies is typically designed with computer aided design (CAD) or computer aided manufacturing (CAM) applications at the design stage. CAD or CAM geometric manipulation may be used to form a surface representation or rendering of the tool on which the composite structure will be formed. Other CAD or CAM surfaces or sheet solids may be used to represent the plies that make up layers of the composite structure.
Known composite design specifications and methods allow for imperfections, such as gaps and overlaps in coverage, at the ply level, but such known specifications and methods do not provide adequate information concerning the detrimental effects that concentrating such imperfections and gap or overlap groupings near each other might have as composite structures are created with multiple layers. Allowable deviations from known design definitions are covered at the ply level by manufacturing specifications. However, known methods do not provide information to compare the cumulative effect such allowable deviations might have at the laminate or stacked level and do not provide information to evaluate the actual tape or tow distribution occurring at the ply level in relation to the plies below and above it in a composite stack or laminate. This can result in composite structures being overdesigned to compensate for potential flaws which results in increased cost and weight. This additional weight reduces the usable payload of the aircraft and decreases fuel efficiency which increases the product operating cost. In addition, the current methods make dimensional tolerance of the stack-up thickness more difficult to predict or control increasing the potential for problems when interfacing to related structures.
Accordingly, it is desirable to provide a method for composite stack analysis for use in the design and manufacture of composite structures that can provide advantages over known methods.